1. Field of the Invention
This invention generally relates to controlling air handling systems for environments. More specifically, this invention relates to precise differential pressure control of the air flow device for an environment's air handling system. The invention is used in manufacturing facilities, hospitals, and research institutions.
2. Description of the Related Art
Today's ever increasing complexity of manufactured products requires an ever higher need to create and maintain a cleaner manufacturing environment. To achieve such an environment, it is necessary to precisely control airflows so that we keep air within a cleaner environment flowing into a dirtier environment. By maintaining a slightly positive differential air pressure between a cleaner area and a dirtier area, air will flow from the cleaner environment to the dirtier environment. However, if the pressure difference is too great between the two environments, more air will try to cross the boundary between the environments and the greater air flow can cause turbulence in the environment. This turbulence can free contaminants in the cleaner environment from the walls or surfaces and can keep airborne contaminants lingering within the cleaner area much longer than would have been the case with a slight pressure due to eddies and non-laminar currents in the air stream. Therefore, the precise control of differential air pressure between two zones or environments can increase cleanliness and keep contaminants from entering the cleaner environment.
There are many manufacturing processes where it is extremely critical to prevent or control airborne contaminants from entering a particular environment so as not to contaminate the final product. For example, consider a typical mini-environment in a semiconductor manufacturing facility such as FIG. 2, which illustrates mini-environment 40 surrounding a typical piece of process equipment. An air handler consisting of fan filter unit 42 forces air through a HEPA filter to filter out airborne particulates. Generally, the air handler is set such that it provides a slightly more positive differential air pressure in the mini-environment than the outside room air pressure. The air flow flowing through the openings and vents in the equipment and the mini-environment is sufficient to keep outside contaminants from entering the mini-environment.
Another such application requiring a very clean or contaminant free environment is found in positive isolation rooms in hospitals. Such rooms are used to protect patients with highly depressed immune systems (a frequent side effect of AIDS for example). In these rooms, it is necessary to create a contaminant free environment that effectively prevents air from entering the room except through HEPA filters. Patients with depressed immune systems are unable to breathe the air from the general hospital environment because contracting an opportunistic infection from the air may prove fatal.
Another set of applications of differential pressure control is to exhaust harmful fumes or contaminated air from an environment. In these applications it is important that outside air flow into the contaminated environment and through a properly equipped exhaust system. An example of this type of application would be a chemical wet bench in a manufacturing plant. As illustrated in FIG. 3, the pressure under the hood would be slightly more negative than the pressure in the room. This would cause the air under the hood and on top of the bench to flow up through the exhaust vent. The exhausted air would then be treated by scrubbers or appropriate treatment systems. As in the above examples, it is important that this differential pressure be maintained in order to ensure that regardless of changing conditions within the room that the air be exhausted at a constant rate. If the exhaust rate is too slow, it can allow fumes to condense on the underside of the hood and then to rain back into adjoining chemical baths thus contaminating process chemicals. If the differential pressure is too high it can cause turbulence in the airflow, possibly disrupting the exhaust such that fumes might actually be more likely to escape into the room.
A similar application in hospitals is the exhausting of respiratory illness rooms. In these rooms, patients with contagious respiratory illnesses like TB, must be isolated from the rest of the hospital and from each other. The pressure in their rooms must be kept more negative than the outside environment. That way, air travels from outside environments, like hallways, through the room and ideally is exhausted near the patient's head to be filtered and then re-released in the air outside and above the hospital. Should the differential between the room and the outside environment become positive, we run the risk of exposing staff and other patients to the illness carried in the air.
A further negative isolation application is in biological safety cabinets used by research institutions to isolate experiments from each other and from research personnel. In these cases, a properly operating hood over the experiment exhausts any airborne products of the experiment preventing cross contamination between experiments under the same hood, or experiments that share the same cabinet at different times. That arrangement will also protect personnel from being exposed to their experiments.
One major problem common to strategies for controlling the air flow in the above applications is that the differential air pressure between the controlled environment (such as in a mini-environment or in a room) and the outside or reference environment constantly changes. Fluctuations in the differential air pressure occur as a result of changes in the air pressure or air flow within the environment due to such events as people accessing the environment, by the operation of the air flow equipment itself; by varying loads on building air handling systems; by the loading of air filters over time; and even by the position and movement of people within the room or environment.
The prior art's attempt to control constantly changing differential air pressure used a variety of open-loop and closed-loop control solutions. Open-loop solutions include on/off switches, and manual speed control units like rheostats. Manual speed control devices are by far the most common form of control devices in air handling systems. Closed-loop solutions include such products as magne-helix and photo-helix type controllers that are passive, non-intelligent, and are generally insensitive at ultra-low pressures; and ultra-low differential pressure sensors connected to a centralized HVAC air control system that relies on distributed sensors and air handling techniques to distribute and balance air in large environments such as entire buildings, which by default make these large systems ill suited to precise local control of room or equipment environments. To account for randomly changing differential air pressure, industry practice is to periodically and manually increase the differential pressure for the controlled environment. Increasing the differential pressure, however, can cause turbulence within the environment, as well as requiring more power to operate the air handling equipment.
The present invention solves the constantly changing differential air pressure in a unique and novel product that continuously monitors the differential pressure between any two environments, and using closed loop control dynamically adjusts the local air handling system to maintain the desired differential pressure setpoint. A comparison of some of the above problems encountered with maintaining a desired differential air pressure setpoint with current air flow control products will illustrate the advantages of the present invention.
U.S. Pat. No. 5,257,958 to Jagers is a pressure override control for air treatment units. The Jagers patent prevents stale air by bringing in enough fresh air into a central HVAC system to induce a positive air pressure inside the building relative to the outside of the building. If the pressure is too low for too long in the building, this product starts the HVAC system to bring fresh air into the building using a bang-bang type of control strategy. This product uses temperature and differential pressure as inputs to control the mixing of fresh and recirculated air as part of the air recirculation--fresh air balance control. Since this product addresses HVAC applications and has a crude and imprecise control, it is not very adaptable to smaller environments or even individual rooms. The Jagers patent addresses the problem of stale air; not precise local control of differential air pressure. The present invention, however, maintains a desired differential pressure setpoint for a local environment. Its focus is on local control of air pressure; not on better balancing of a global air handling system, although its inclusion in a global system will help better manage global airflows by handling critical areas more discretely than is done currently.
Another current control product, U.S. Pat. No. 5,115,728 to Ahmed et al., controls the differential pressure of rooms with laboratory fume hoods. This patent maintains a constant face velocity for a fume hood by monitoring the position of the fume hood sash (variable opening or moveable door) and sending control signals to an external HVAC controller to help it maintain a desired hood and room pressure. Like the Jagers patent above, this patent relies on the building's HVAC system to maintain control over these differential pressures. The position of the hood's sash is critical for this patent to maintain a low pressure by only using negative pressure. Additionally, this patent then connects multiple fume hoods together and transmits the sash information along with room information to a central building controller. The central building controller must then attempt to meet the control requirements of the individual units as best it can by making tradeoffs and adjustments for each unit, sometimes at the expense of other units.
The present invention, however, uses negative and positive pressure to maintain a set differential pressure across a large range of pressures for a wide variety of applications, not just for fume hoods. The present invention can control a variety of air flow devices such as fans, pumps, compressors, dampers, valves, and the like, directly. By controlling the airflow device directly, this invention does not need to send control signals to the HVAC system, and can control the pressure of the room without the help of an additional controller. The present invention is not merely a repackaging or an integration issue. It is a fully integrated local controller having the means to sense and to affect local control of an environment based on desired parameters. The present invention can operate independently of a large centralized air handling system or be used in conjunction with such a system in order to meet critical air handling requirements. When used in a hybrid system with an HVAC system, the present invention provides precise control over a local environment, leaving the big picture to the larger system.
Returning to the multiple fume hoods of the Ahmed patent, we can use the present invention to control multiple fume hoods by having an individual unit of the present invention on each fume hood. A separate unit provides local control of the differential pressure of that individual fume hood. Another unit of the present invention would separately control the room pressure using either positive or negative pressure, and would ignore the controllers on the fume hoods by monitoring the room pressure directly. Controlling the room pressure in this manner will compensate for any and all air leaks from the room including doors and windows, instead of just fume hoods.